Project Summary Our proposed research is aimed at understanding the molecular mechanisms that underlie homologous chromosome segregation during reproductive cell formation. Meiosis is the specialized cell division cycle that partitions the two homologous copies of every chromosome (homologs) to separate daughter nuclei, effectively reducing chromosome ploidy. Errors in chromosome segregation lead to aneuploid reproductive cells that carry too many or two few chromosomes. Key to the success of homolog segregation is the prior establishment of transient but stable associations between chromosomes; in most organisms a large number of these links are formed by crossover recombination events. How crossover associations are efficiently generated between every chromosome pair during meiosis remains poorly understood, but for most organisms it is clear that the process involves an exquisite coordination between large-scale chromosome movements and local DNA repair processes. Assembly of a conserved protein-rich structure, the synaptonemal complex (SC), along the length of homologous partner chromosome axes mediates an intimate alignment and forms the context in which DNA repair intermediates mature. SC has long been associated with successful crossover recombination, and although our recent research demonstrated that the SC structure per se is dispensable for crossing over in budding yeast, we also showed that the SC building block component, Zip1, has a genetically separable function in promoting crossovers. A structure-function study identified twenty N terminal amino acids that play a key role in both Zip1?s specialized crossover function and in its capacity to assemble SC. Our data indicate that Zip1?s N terminal residues carry out their crossover function by interfacing with the pro-crossover protein and E3 SUMO ligase, Zip3. Moreover, in collaboration with the Davies lab at Newcastle University, we have begun a structural analysis of two additional building blocks of the SC that likely interface with Zip1?s N terminus during SC assembly. Our proposed experimets are designed to support a uniquely rich training environment for several undergraduates and one graduate student at Weselyan University; these students will undertake primarily molecular genetic. cytological and biochemical approaches in yeast to identify and characterize the factors that promote the coordinated landmark meiotic processes of interhomolog crossover recombination and SC assembly, via an interface with the N terminus of Zip1.